(1) Field of the Invention
The present invention relates to a gradation correcting apparatus by which the gradation of video signals of such devices as television receivers, video-tape recorders, video projectors or others are automatically corrected at the optimum level according to the characteristics of the input image. More specifically, this invention relates to a gradation correcting apparatus which can be effectively incorporated into various video devices in order to display a small sub-display screen area, such as window, on the main display screen area (what is called "Picture in Picture").
(2) Prior Arts
In order to improve the TV image quality, various gradation correcting apparatii dependent on the characteristics of input images had been developed. A conventional and typical gradation correcting apparatus developed in the past is described below by referring to FIGS. 6 to 8.
A block diagram a conventional gradation correcting apparatus is shown in FIG. 6 wherein timing pulse generator 1 generates window pulses by which the range of the histogram on the image screen is determined according to both the horizontal and the vertical synchronization signals.
Histogram memory 2 memorizes the distribution of luminance levels of input video signals which are in a range determined by the pulses generated by said timing pulse generator 1. Lookup table arithmetical processing circuit 3 performs accumulative additions of the histograms stored in the histogram memory 2, and normalizes each data so that the maximum accumulative frequency becomes equal to the maximum value of the output luminance signal.
Lookup table memory 4 stores the data normalized by the lookup table arithmetical processing circuit 3, and from which the correction signal according to the luminance level of input video signal is read out. Delay circuit 5 delays the input video signal, adder 6 adds the correction signals obtained by the lookup table arithmetical circuit 3 to the output of the delay circuit 5.
The operations and relationship of the above explained elements constituting the above-shown gradation correcting apparatus are explained below.
FIG. 7 illustrates a window pulse obtained by said timing pulse generator 1. When a histogram has to be taken within a domain on the screen shown in FIG. 7(a), a window signal such as shown in FIG. 7(b) is derived. FIG. 7(c) shows a horizontal partial enlargement of the signal. The sampling of the histogram is performed only when this window pulse takes a high-level.
FIG. 8 shows graphically the operations of the luminance conversion performed by the conventional apparatus. In this operation, a histogram wherein the input luminance levels are divided into suitable numbers as the one shown In FIG. 8(a), is produced.
The frequency distributions of the divided luminance levels are stored in the histogram memory 2 while the content of the memory is reset at a predetermined interval in order to reset the previously stored data. This interval is generally set at an interval which equals a single vertical scanning period or an integer multiple.
Then, the lookup table arithmetical processing circuit 3 performs an accumulation of the histogram data, and it derives a normalizing coefficient which makes the accumulated maximum value equal to the maximum value of the output luminance level, and the processing circuit multiplies each data of the accumulative histogram to the derived normalizing coefficient. The results of this are stored in the lookup table 4.
FIG. 8(b) shows these operations. Thee content of the lookup table memory 4 is read out according to the luminance level of the input signal, and, as shown in FIG. 8(c), the difference between the output and input luminance levels is outputted as a correction signal. The correction of the gradation is then performed by adding the correction signal to the input signal which is delayed by means of the delay circuit 5 using the adder 6.
However, a below-described problem is inevitable with the gradation correcting apparatus construction shown above when it is incorporated in a TV showing double displays such as the one shown In FIG. 9(a) (what is called "Picture in Picture") wherein a sub-display signal is added to a main display signal.
When the main-display is showing a low luminance scene while the sub-display is showing a high luminance scene, the distribution of the accumulative histogram stored in the lookup table contains a considerably high proportion of the low luminance components, as shown in FIG. 9(d).
Therefore, the correction signal will take such a form shown in FIG. 9(e). Thus, the correction is made to bring the luminance of the sub-display toward a higher level, and therefore, the luminance of the sub-display of which luminance level was originally high will be corrected toward a still higher level, bringing a definitely undesirable TV-viewing condition.